The circulating concentrations of antibodies directed against tetanus toxoid were determined by means of the mouse protection test in the serum samples of 2554 patients with an age between 60 and 98 years. Additionally, a part of the sera was tested by a radioimmunologic procedure. Immunity (neutralizing antitoxin titer greater than or equal to 0.01 International Units/ml serum) was only demonstrable in a small percent of the samples (15.3%). Compared to women, a larger percentage of men were found to be protected. Active immunization of aged persons (60-93 years old) resulted in the finding that usually also during senescence powerful immunity may develop following suitable vaccination.